axi_dmac: assert xfer_request only when ready

If the req_valid asserts faster than the ID gets synchronized over we
assert the xfer request without being ready to accept data.
This can lead to overflow assertion when using a FIFO like interface.

library/axi_dmac/data_mover.v

@@ -105,7 +105,7 @@ wire last_load;
 wire last;
 wire early_tlast;
 
-assign xfer_req = active;
+assign xfer_req = active & pending_burst;
 
 assign response_id = id;